FIG. 1 shows a conventionally used spark plug 9 for an internal combustion engine. For example, the spark plug 9 is used as a means for igniting an air-fuel mixture introduced into a combustion chamber of an internal combustion engine such as of a passenger car.
The spark plug 9 includes a center electrode 94 and a ground electrode 95. The ground electrode 95 has an end fixed to a housing 92, while being crooked to bring the other end to a position facing the center electrode 94, to form a spark discharge gap 911 in relation to the center electrode 94. The ground electrode 95 is provided with a projection portion 96 which is projected toward the spark discharge gap 911 (see Patent Document 1). Thus, as shown in FIG. 2 by (A) and (B), discharge is caused in the spark discharge gap 911 to ignite an air-fuel mixture by the discharge. In the figure, a reference E indicates a discharge spark formed by the discharge, a reference F indicates a flow of the air-fuel mixture, and reference I indicates a flame.
Recently, various lean-burn internal combustion engines are developed to enhance fuel efficiency. In lean burn, the flow speed of the air-fuel mixture in the combustion chamber is designed to be high in order to retain ignitability to the air-fuel mixture. On the other hand, in the spark discharge gap, ignition performance depends, to a large extent, on the positional relationship of the ground electrode with respect to the direction of the gas flow in the combustion chamber. For this reason, the position of the ground electrode is adjusted relative to the direction of the flow of the air-fuel mixture. In this regard, a technique is suggested with which a spark plug is mounted to an internal combustion engine so that the ground electrode will not be located upstream or downstream in the gas flow (see Patent Document 2).
In order to achieve good combustion, some lean-burn internal combustion engines mentioned above use a so-called in-cylinder direct injection system in which an air-fuel mixture is directly injected into the combustion chamber. Such an in-cylinder direct injection system ensures ignitability by enriching the air-fuel mixture near the spark discharge gap. Therefore, the spark plug is fouled by carbon which is induced by incomplete combustion, i.e. carbon adheres to a porcelain tip portion of the spark plug, creating an electrically conductive state. This raises a problem that discharge is not appropriately obtained in relation to the ground electrode. As a measure against such a problem, a spark plug 90 as shown in FIG. 3 is suggested. The spark plug 90 is configured by a main ground electrode 951 that forms a main gap 912 and sub ground electrodes 952 that form a sub gap 913 to thereby enhance resistance to carbon fouling (see Patent Documents 3 and 4).